The present embodiments relate to standing wave traps.
Magnetic resonance devices for examining objects or patients using magnetic resonance tomography (MR) are known from DE10314215B4, for example.
Running wiring or cables in the vicinity of transmit antennas of an MR system causes standing waves to be stimulated. The standing waves may also propagate on an outer conductor of coaxial cables. A known method for suppressing standing waves is the use of chokes on ferrite cores. This method may be used, for example, in the case of switched-mode power supplies, since the sources of interference extend over a broad frequency spectrum.
Ferrite cores are problematic in the vicinity of antennas and, for example, in the case of transmit antennas in MR systems.
If the standing waves are not suppressed, the following problems may occur:                coupling-in of sources of interference or destruction of connected electronic components;        increased power requirement for the transmit antenna, since part of the transmit power drains away as the standing wave; and        the standing wave may lead to local field enhancements.        
Standing wave filters or traps may be fitted closely to the “source of interference.” If standing wave traps are used for MR transmit antennas, the following known solutions exist in the prior art by reason of the constraints (e.g., static magnetic field, small space requirement, dielectric strength):
1. A “balun” according to FIG. 2: A cylindrical (or also cuboidal or hexagonal) conductive structure is pushed over the cable and connected at one end to the outer conductor of the cable. Given a length L of the balun, this arrangement acts as a standing wave trap at the frequency F=c/(4*L), where c is the propagation speed in the medium between the cable and the balun. If the blocking effect is to be realized at a different frequency, this may be achieved by adding capacitors.
2. “Wound standing wave trap” according to FIG. 3: The cable is wound to form a coil, and a capacitor is arranged in parallel with the coil. In this way, a parallel resonant circuit is formed. The parallel resonant circuit exhibits a high impedance at the resonant frequency of the parallel resonant circuit and consequently, suppresses the propagation of standing waves.